Process or using subterranean produced fluids for hydraulic fracturing with cross-linked gels while providing elimination or reduction of formation clay stabilizer chemicals

ABSTRACT

A system and process for defining, blending and monitoring fresh water with subterranean produced formation fluids, with particular constituents of the blended waters being controlled for proper use in gel-type hydraulic fracturing operations. On-site measurements and calculations of clay stabilization replacement, through a Potassium Chloride (KCl) Equivalency calculation, provide feedback on water constituent adjustments that may be needed just prior to the gel-based hydraulic fracturing process. This assures adequate gel cross-linking times, delayed gel cross-linking times, and clay stabilization in the formation to be fractured.

This application is a continuation of U.S. patent application Ser. No.14/451,654, filed Aug. 5, 2014, which claims benefit of and priority toU.S. Provisional Application No. 61/862,243, filed Aug. 5, 2013, by MarkPatton, et al., and is entitled to those filing dates for priority. Thespecifications, figures, appendices and complete disclosures of U.S.Provisional Application No. 61/862,243 and U.S. patent application Ser.No. 14/451,654 are incorporated herein by specific reference for allpurposes.

FIELD OF INVENTION

This invention relates to a system and process for hydraulic fracturing.More specifically, this invention relates to a system and process forcross-linked gel-based hydraulic fracturing during the completionprocess of oil and gas wells.

BACKGROUND OF INVENTION

Hydraulic fracturing of subterranean formations requires an enormousvolume of water to adequately transfer pressure and to provide atransport media for delivering the fracture proppants into theformation. Examples of hydraulic fracturing methods and proppants aredisclosed in U.S. Pat. Nos. 7,931,966; 7,938,185; and 8,061,424, all ofwhich are incorporated herein by specific reference in their entiretiesfor all purposes. As water becomes scarcer and scarcer in the world,solutions for utilizing or reusing fluids generated or “produced” fromone or more completed wells in a gel-based fracture fluid design of asubsequently to-be-fractured well has been attempted with littlesuccess.

Accordingly, what is needed is an improved system for using producedfluids in hydraulic fracturing.

SUMMARY OF INVENTION

In various embodiments, the present invention comprises a system andprocess for defining, blending and monitoring fresh water (FW) withsubterranean produced formation fluids (PF), with particularconstituents of the blended waters being controlled for proper use ingel-type hydraulic fracturing operations. The invention compriseson-site measurements and calculations of clay stabilization replacement,through a Potassium Chloride (KCl) Equivalency calculation, to providefeedback on water constituent adjustments that may be needed just priorto the gel-based hydraulic fracturing process. This assures adequate gelcross-linking times, delayed gel cross-linking times, and claystabilization in the formation to be fractured. Real-time measurementsinclude, but are not limited to, blend ratios (volumetric), boronlevels, chloride levels, and Total Dissolved Solids (TDS). Levels ofchlorides and boron are measured in the waters to be blended, and thenthe volumetric ratios of blend can be calculated to successfully allowcross-linking of borate cross-linked gels during real-time hydraulicfracturing operations. By applying specific calculations, as describedbelow, adjustments in volumetric ratios can then be performed forreliable fluid design.

In addition, this invention comprises methods for calculating andmonitoring of Potassium Chloride (KCl) Equivalency in order to determinea set or range of produced fluid volume percentage blend limits. Theestablishment of blend limits then enables the end-user to eliminate orgreatly reduce the volume of clay stabilizer chemical that might be usedin the absence of this blending process. During the blend monitoringprocess, confirmation samples are obtained at relevant sample points andreal-time TDS values of the blended water are measured and documented asan indicator that (a) the chlorides, and thus water quality, of eitherthe Fresh Water or Produced Fluid are rising or falling; (b) the ratioof the water blend, as defined by the KCL Equivalency calculation, ischanging by possible speed (RPM) changes in the fluid pumps or that oneof the pumps is failing to transfer its proper volumetric load; or (c)some combination of both (a) and (b).

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of the blending process in accordance with anembodiment of the present invention.

FIG. 2 is an example of a graph of volume of fresh water and producedfluid blended at different stages.

FIG. 3 is an example of a graph of boron concentration in produced fluidand blended water at different stages.

FIG. 4 is an example of a graph of chloride concentration in producedfluid and blended water, and KCl Equivalent, at different stages (withKCl target of 3% to 6%).

FIG. 5 is a table of calculated blend rates of produced fluid to meetspecified KCL Equivalents for different stages.

FIG. 6 is an example of a graph of calculated blend rate to meetdifferent KCL Equivalents.

FIG. 7 is an example of a graph of calculated flow rates to meetdifferent KCL Equivalents.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

In various embodiments, the present invention comprises a system andprocess for defining, blending and monitoring fresh water (FW) withsubterranean produced formation fluids (PF), with particularconstituents of the blended waters being controlled for proper use ingel-type hydraulic fracturing operations. The invention compriseson-site measurements and calculations of clay stabilization replacement,through a Potassium Chloride (KCl) Equivalency calculation, to providefeedback on water constituent adjustments that may be needed just priorto the gel-based hydraulic fracturing process. This assures adequate gelcross-linking times, delayed gel cross-linking times, and claystabilization in the formation to be fractured. Real-time measurementsinclude, but are not limited to, blend ratios (volumetric), boronlevels, chloride levels, and Total Dissolved Solids (TDS). Levels ofchlorides and boron are measured in the waters to be blended, and thenthe volumetric ratios of blend can be calculated to successfully allowcross-linking of borate cross-linked gels during real-time hydraulicfracturing operations. By applying specific calculations, as describedbelow, adjustments in volumetric ratios can then be performed forreliable fluid design.

In addition, this invention comprises methods for calculating andmonitoring of Potassium Chloride (KCl) Equivalency in order to determinea set or range of produced fluid volume percentage blend limits. Theestablishment of blend limits then enables the end-user to eliminate orgreatly reduce the volume of clay stabilizer chemical that might be usedin the absence of this blending process. During the blend monitoringprocess, as seen in FIG. 1, confirmation samples are obtained at SamplePoint 3 (SP3) and real-time TDS values of the blended water are measuredand documented as an indicator that (a) the chlorides, and thus waterquality, of either the Fresh Water or Produced Fluid are rising orfalling; or (b) the ratio of the water blend, as defined by the KCLEquivalency calculation, is changing by possible speed (RPM) changes inthe fluid pumps or that one of the pumps is failing to transfer itsproper volumetric load; or (c) some combination of both (a) and (b).

FIG. 1 shows an example of a blending processing flow in accordance withone exemplary embodiment of the present invention. Pump 1 is associatedwith the Fresh Water stream, and Pump 2 is associated with the ProducedFluid stream. Prior to blending the fresh water and produced fluid,samples are collected at the storage facilities for each fluid. Thesestorage locations could be pits or tanks or other similar means known inthe art for storing fluids. In one embodiment, these samples may beanalyzed at an on-site lab to determine chloride and boron levels inparts per million (ppm), although other measurement units may be used.

The process begins by turning on Pumps 1 and 2 and adjusting the speedof each pump (e.g., RPM1, RPM2) to match pre-defined ratios of FreshWater (FW) to Produced Fluid (PF) need for a blended outlet flow rate(Q_(TT)), as follows:

% FW:% PF 50:50 60:40 75:25 80:20 85:15 90:10For example, if a total flow rate (Q_(TT)) out of the blending processis defined as needing to be 40 barrels/minute (BPM), the Current FlowRate of Fresh Water (Q_(CF)) and Current Flow Rate of Produced Fluid(Q_(C)p), as measured by Flow Meter 1 (FM1) and Flow Meter 2 (FM2),would be set as follows by adjusting the speed of each pump:

% FW:% PF Q_(CF) (bpm) Q_(CP) (bpm) 50:50 20 20 60:40 24 16 75:25 30 1080:20 32 8 85:15 34 6 90:10 36 4

As each flow rate ratio is stabilized, two sets of samples of theblended water are obtained at Sample Point 3 (SP3) after the blendingchamber. One set of samples is analyzed on-site for chloride and boronlevels, and calculations are done to provide KCl Equivalency values (in%) for each of the blended sample ratios. This enables the end-user toidentify a recommended blend ratio based upon the targeted KClEquivalency range for clay stabilization (as disclosed in more detailedin the attached appendix).

The second set of samples may be sent off-site for independent gelcompatibility testing to determine the desired blend ratio based uponthe gel products of choice and the various levels of chlorides and boronin the blended water. During the period of selection of the blend ratiofor the fracturing operation, circulation of the unblended raw producedfluid may be performed to provide homogenous chloride levels once thefracture operation stages begin.

Upon determination of the desired FW:PF blend ratio, the waters/fluidsare blended on a stage-by-stage volumetric “batch” basis to prepare theblended water for the hydraulic fracturing operation. During each stageof blending, the volumetric flow rates and volumes are recorded for eachinlet flow stream (FW and PF) to hold the blend ratio steady, as wasdefined by the KCl Equivalency target but subject to the predefinedboron level maximum (B_(T)) in the blended water. For each stage “batch”of blended water, samples are obtained at the blending chamber outlet orat each storage tank to define and check chloride and boron levels, andto recalculate the actual KCl Equivalency value along with a check onthe boron level maximum value in case the fracture fluid design needs tobe adjusted for adequate cross linking delay times and gel constituentmixtures. In addition, samples of the unblended produced fluid (PF) areobtained and checked at the on-site lab for fluctuations in chloride andboron levels. Examples of this test data are shown in FIGS. 2-5, and maybe used for adjustment of blend ratios as needed.

The pre-stage “batch” blending process is repeated for each stage of thegel-based hydraulic fracturing operation to assure compatibility withrespect to KCl equivalency targets and maximum boron levels (B_(T)) inthe blended waters.

The calculations for the hydraulic fracturing fluid blending process areas follows. For the calculation of KCL (Potassium Chloride) Equivalent,one molecule of KCl consists of one molecule of K (Potassium) and onemolecule of Cl (Chloride). The molecular weight for K is 39 g/mol, whilethe molecular weight for Cl is 35.5 g/mol, so the molecular weight forKCl is 74.5 (39+35.5) g/mol. Assuming the density of the fluid is at1000 g/L, for a known chloride concentration C_(Cl) (ppm or mg/L), theKCl equivalent can be calculated based on the following equation:

${{KCl}\mspace{14mu}{Equivalent}\mspace{11mu}(\%)} = {{\frac{74.5\frac{g}{mol}}{35.5\frac{g}{mol}} \times C_{Cl}\frac{mg}{L} \times 100} = {{\frac{74.5}{35.5} \times C_{Cl} \times \frac{1\mspace{14mu}{mg}}{1\mspace{14mu} L \times 1000\frac{g}{L} \times 1000\frac{mg}{g}} \times 100} = {0.00020986\mspace{11mu} \times {C_{Cl}(\%)}}}}$

In a specific gel-frac design process for hydraulic fracturing, thefollowing parameters are established for a particular design:

-   -   Target Flow Rate Total, Q_(TT)    -   Target KCl Equivalent, KCl_(T)    -   Target KCl Equivalent Maximum, KCl_(max)    -   Target KCl Equivalent Minimum, KCl_(min)    -   Target Boron Level Maximum, B_(T)        The following parameters are monitored in real-time to ensure        the quality of the water meets the above targets:    -   Current Flow Rate of Produced Fluid, Q_(CP)    -   Current Flow Rate of Fresh Water, Q_(CF)    -   Current Fresh Water Chloride Level, C_(CF)    -   Current Produced Fluid Chloride Level, C_(CP)    -   Current Blended Water Chloride Level, C_(CB)    -   Current Fresh Water Boron Level, B_(CF)    -   Current Produced Fluid Boron Level, B_(CP)    -   Current Blended Water Boron Level, B_(CB)

After obtaining these parameters, the Current Blend Rate Produced(P_(CP)) and Current Blended Water KCl Equivalent (KCl_(CB)) can besolved through the following equations:

$P_{CP} = {\frac{Q_{CP}}{Q_{CF} + Q_{CP}} \times 100(\%)}$KCl_(CB) = 0.00020986   × C_(CB)(%)

The blending process may lead to elimination (such as precipitation) orgeneration (such as dissolution from precipitation) of boron or chlorideinto the blended water solution. Blending coefficients (R_(B) and R_(C))are defined to evaluate the effects of elimination or generation onboron and chloride during the blending process.

$R_{B} = \frac{B_{CB}}{{B_{CF} \times \left( {1 - \frac{P_{CP}}{100}} \right)} + {B_{CP} \times \frac{P_{CP}}{100}}}$$R_{C} = \frac{C_{CB}}{{C_{CF} \times \left( {1 - \frac{P_{CP}}{100}} \right)} + {C_{CP} \times \frac{P_{CP}}{100}}}$If R_(B) or R_(C)<1, part of the boron or chloride is eliminated(precipitated) from the solution through the blending process. If R_(B)or R_(C)=1, boron or chloride level in the solution is not affected theblending process. If R_(B) or R_(C)>1, part of the boron or chloride isdissolved (from solid) into the solution through the blending process.

In the control process, KCl has the primary priority over boron. Inanother words, the adjustment of the blend rate aims at achieving thetarget KCl equivalent (KCl_(T)) by adjusting the fresh water andproduced water flow rates, Q_(CF) and Q_(CP) respectively to maintain aconstant ratio. If the boron level after the adjustment (B_(A)) is overthe maximum boron allowance (B_(T)), notification can be made to theuser or fracturing operator that the process needs boron inhibitor toensure enough delay for the cross linking of the gel.

To meet the target KCl equivalent, the blend rate should follow thefollowing equation:

${\left. \left\lbrack {{C_{CF} \times \left( {1 - \frac{P_{TP}}{100}} \right)} + {C_{CP} \times \frac{P_{TP}}{100}}} \right) \right\rbrack \times R_{C} \times 0.00020986}\mspace{11mu} = {KCl}_{T}$Solve the equation for P_(TP),

$P_{TP} = {\frac{\frac{{KCl}_{T}}{R_{C} \times 0.00020986} - C_{CF}}{C_{CP} - C_{CF}} \times 100}$All other unknown parameters will then be solved by the followingequations:

$Q_{TP} = {Q_{TT} \times \frac{P_{TP}}{100}}$ Q_(TF) = Q_(TT) − Q_(TP)$B_{A} = {\left\lbrack {{B_{CF} \times \left( {1 - \frac{P_{TP}}{100}} \right)} + {B_{CP} \times \frac{P_{TP}}{100}}} \right\rbrack \times R_{B}}$

In several embodiments, notifications are provided during operation ofthe system as follows:

Current Status KCl_(CB) > KCl_(max), B_(CB) > B_(T) KCl Over UpperLimit; Boron Over Upper Limit KCl_(T) < KCl_(CB) ≤ KCl_(max), KCl OverTarget Limit; Boron Over B_(CB) > B_(T) Upper Limit KCl_(CB) = KCl_(T),KCl At Target Limit; Boron Over B_(CB) > B_(T) Upper Limit KCl_(min) ≤KCl_(CB) < KCl_(T), KCl Under Target Limit; Boron Over B_(CB) > B_(T)Upper Limit KCl_(CB) < KCl_(min), KCl Under Lower Limit; Boron OverB_(CB) > B_(T) Upper Limit KCl_(CB) > KCl_(max), KCl Over Upper Limit;Boron Under B_(CB) ≤ B_(T) Upper Limit KCl_(T) < KCl_(CB) ≤ KCl_(max),KCl Over Target Limit; Boron Under B_(CB) ≤ B_(T) Upper Limit KCl_(CB) =KCl_(T), KCl At Target Limit; Boron Under B_(CB) ≤ B_(T) Upper LimitKCl_(min) ≤ KCl_(CB) < KCl_(T), KCl Under Target Limit; Boron UnderB_(CB) ≤ B_(T) Upper Limit KCl_(CB) < KCl_(min), B_(CB) ≤ B_(T) KClUnder Lower Limit; Boron Under Upper Limit

Actions to Take Blend Water at Q_(TP) bbls/min Produced Fluid and Q_(TP)bbls/min Fresh Water

Projected Status B_(A) ≤ B_(T) KCl At Target Level; Boron Under UpperLimit B_(A) > B_(T) KCl At Target Level; Boron Over Upper Limit, NeedBoron Inhibitor

Graphs for blend rate and flow rate are automatically generated as shownin FIGS. 6 and 7. The user can use those graphs as quick references whenor if they desire a different target KCl Equivalent.

Thus, it should be understood that the embodiments and examplesdescribed herein have been chosen and described in order to bestillustrate the principles of the invention and its practicalapplications to thereby enable one of ordinary skill in the art to bestutilize the invention in various embodiments and with variousmodifications as are suited for particular uses contemplated. Eventhough specific embodiments of this invention have been described, theyare not to be taken as exhaustive. There are several variations thatwill be apparent to those skilled in the art.

What is claimed is:
 1. A system for blending fluids for hydraulicfracturing, comprising: a blending chamber for blending fresh water andproduced fluid to produce blended water; a fresh water inlet lineproviding fresh water at a fresh water flow rate to the blendingchamber; a produced fluid inlet line providing produced fluid at aproduced fluid flow rate to the blending chamber; and a blended wateroutlet line adapted to remove blended water from the blending chamber;wherein the relative flow rates of fresh water and produced fluid aredetermined based upon a target Potassium Chloride Equivalency value orrange for the blended water.
 2. The system of claim 1, said blendedwater outlet line comprising an outlet line sample point, said freshwater inlet line comprising a fresh water inlet line sample point, andsaid produced water inlet line comprising a produced water inlet linesample point.
 3. The system of claim 2, further wherein the relativeflow rates of fresh water and produced fluid are determined based uponthe chloride concentration in the fresh water and the chlorideconcentration in the produced fluid.
 4. The system of claim 3, whereinthe flow rates of the fresh water and produced fluids are set to achievethe target Potassium Chloride Equivalency value or range in the blendedwater.
 5. The system of claim 2, wherein an actual Potassium ChlorideEquivalency value for the blended water is determined from one or moresamples obtained at the outlet line sample point.
 6. The system of claim5, wherein the actual Potassium Chloride Equivalency value is determinedbased upon the chloride concentration in said one or more samplesobtained at the outlet line sample point.
 7. The system of claim 6,wherein the flow rates of the fresh water and produced fluids areadjusted if the actual Potassium Chloride Equivalency value is notsubstantially equal to the target Potassium Chloride Equivalency valueor within the target Potassium Chloride Equivalency range.
 8. The systemof claim 6, wherein the Potassium Chloride Equivalency value iscalculated as 0.00020986 times chloride concentration.
 9. The system ofclaim 2, further wherein an actual boron concentration for the blendedwater is determined from one or more samples obtained at the outlet linesample point.
 10. The system of claim 1, wherein the blended water isproduced on a batch basis for a stage in a hydraulic fracturingoperation.